1. Field of the Invention
The present invention generally relates to gas filtration and purification. In particular, the invention is a novel integral, all-metal gas filter and purifier combination, with high efficiency, low outgassing, and superior purification characteristics that make it useful as a point-of-use getter-filter combination for semiconductor process gases.
2. Description of the Prior Art
Semiconductor manufacturing is constrained by the limitations of purity. In the chemical vapor deposition of the dopant atoms to silicon, a critical aspect of the process involves the absence of any homogeneous or heterogeneous impurities. For example, the presence of minute particle or trace oxygen or moisture impurities can significantly damage an entire silicon wafer representing many dollars of potential end-product. To that end, an entire industry has developed concerned with the filtering and purifying of the gases that may come into contact with the semiconductor product during its manufacture.
Clean rooms equipped with HEPA (High Efficiency Particulate Attenuator) filters are the first line of defense. Process equipment is located within "clean rooms" that are filled with carefully filtered air. The design of the equipment itself endeavors to minimize particle shedding, outgassing, and contamination from the materials used to transport and deliver high-purity gases such as argon, nitrogen, silane, arsine, hydrochloric acid and phosphine. An important component in the gas delivery is the filter which insures that particulate contamination does not reach the point where the gas is discharged onto the work (point-of-use). These filters must not only remove any particulate material, but also must not add any gaseous contamination to the high purity gases. In addition, the gas delivery systems must also be as compact as possible to eliminate contamination, both particulate and gaseous, which might arise from either the installation of such systems, or the normal wear associated with usage. Therefore the filters must not only remove particulate material and not be a contributor of gaseous impurities, but they must also be as compact as possible and have small internal and filter volumes.
Various filters are used for filtration of such gaseous fluids to insure ultra high levels of purity in terms of particulate contamination. These include: organic membrane filters, ceramic filters, filters formed from porous metal structures and filters formed from metal fibers. Although some of these various filter media are capable of providing particulate contamination control to levels less than one part per million or greater, they are characterized by large filter areas. Due to the large flow area required to sustain flow at reasonable pressures and maintain low face velocities to insure particulate retention, gaseous impurities such as moisture, oxygen and especially hydrocarbons are often present at detectable levels (parts per million). This contamination can occur during manufacture of the filter, installation of the filter when it is exposed to an atmosphere other than a high purity gas, or even as a result of outgassing from the material in which the filter is packaged. In addition, large filter volumes require relatively larger housings to contain them. This in turn results in a greater likelihood of contamination due both to installation and usage and the need for larger gas delivery systems to fit the filters.
Present metal filters are made from metals that include stainless steel, nickel, or nickel alloy sintered-powder types such as the Wafergard.RTM. II SF (Millipore Corporation, Bedford, Mass.), the Ultramet-L.sup.TM (Pall Corp, Glen Cove, N.Y.), and the Molt GasShield.TM. line of filters (Mott Metallurgical Corporation, Farmington, Conn.) (see U.S. Pat. No. 5,114,447 (Davis)). Such filters, being all metal, exhibit low outgassing, high efficiency, corrosion and temperature resistance, and high structural strength with low porosity and gas throughput. The low porosity has continued to be a drawback for typical sintered metal powder filter elements. Porosities for the above filters range at best from 40 to 44%, limiting the flow-through characteristics of these filters. The low porosities are inherent in the processes used to manufacture sintered metal powder filters. In such a process, the powders are typically compacted into a mold to form a "green form," then sintered to join the metal particles together to impart the necessary strength. The final filter elements (or "membranes") may be cut from a fiat sintered sheet of metallic powder, or molded into the final shape in the molding step. The temperatures at which the sintering is conducted are critical factors in determining the final porosity. Higher temperatures lead to increased strength, but lower porosity; lower temperatures lead to decreased strength and higher porosity. Until now, the final porosity was limited to about 45% in the sintered metal powder art.
Removal of trace contaminant gases is also an issue in semiconductor manufacturing. Current gas purification devices utilized in the microelectronics industry consist of two separate modules integrated into one package, i.e. an upstream purifier (containing getter alloys, organometallic or inorganic resin-based or reactive micro matrix materials) for removal of molecular contaminants followed by a downstream particle filter for removal of particles. These particles may already have been present in the gas stream and/or were generated by the purifier. Representative examples of these products include purifiers sold by SAES Pure Gas, Ultrapure, Semi-gas and Millipore. For example, Millipore sells a point-of-use purifier called the Waferpure.RTM. Mini XL that consists of two separate pans, a small quantity of purification material and a stainless steel filter, contained within a single stainless steel housing that measures five inches in length and one inch in diameter. Other larger purifiers contain two separate housings connected in series, i.e. an upstream housing that contains the purification material followed downstream by a suitable gas filter. Purifers that consist of separate purification and filtration modules, regardless of how packaged, can only function properly in one flow direction. Reverse flow or sudden back diffusion might result in the release of fine particles from the purifier bed through the retaining frit and into the gas line.
Getters have been used to absorb or chemically bind trace amounts of gases such as oxygen, carbon monoxide, hydrogen and moisture, thereby maintaining vacuum in sealed devices, and for purifying inert gases. In the past, zirconium and titanium sponge have been used, but must operate at elevated temperatures (700.degree.-900.degree. C.). Other non-evaporable gettering alloys have been developed which are made from metal alloy powders or their hydrides, and have small size (less than 125 .mu.m) and lower operating temperatures (350.degree. C.). The powders are packed into columns, admixed with a supporting bed, and used in-line to absorb or catalytically remove trace gases. These packed beds have the chief drawback of the generation of fines due to mechanical abrasion of the particles under conditions of normal pressure and flow fluctuations. Getter alloys have included combinations of Zr, AI, Ni, Fe, Ti, Ta, Th, Hf, Nb, and uranium. U.S. Pat. No. 4,312,669 (Boffito, et al.) discloses a ternary alloy of Zr-V-Fe that may be pressed into pellets or attached to a support, or alternatively chemically bound and sintered. The finely divided alloy powders are cast or pressed into structures, or they are combined with a support such as in U.S. Pat. No. 5,242,559 (Giorgi), which describes a getter electrolytically bound to a support such as a metal wire or strip, or metal-coated ceramic, in combination with an antisintering agent. The bound getter powder, antisintering agent and support are then heat treated (sintered) under vacuum to make a porous surface coating. However, these coatings are not useful for forming filter-type structures because they are restricted to having a support surface. In addition, since sintering is aided with a binder, it is possible that portions of the binder may slough off during a point-of-use application, again generating fines that may find their way into the process stream.
A recent development in this area is a reactive carbon/ceramic membrane for filtration and purification of gases, which has a reactive metal and is applied as a layer. U.S. Pat. No. 5,196,380 (Shadman) discloses a reactive membrane for removal of homogenous and heterogenous impurities from gases, the reactive membrane comprising a porous ceramic or carbon substrate upon which a carbon layer is deposited, and a layer or layers of reactive reduced metal on the carbon, the metal being selected from the group consisting of manganese and alkali metals. The metal is first deposited in non-reduced form and subsequently reduced. Chemical vapor deposition and solution deposition techniques are disclosed for depositing both the carbon and the metal. This device, however, has the following manufacturing and performance problems. For the filter element utilized as the starting material, the pore size needs to be slightly larger than the final product since layers of material will be deposited on the membrane and thereby decrease the pore size. This creates potential manufacturing difficulties for reproducibility and pore plugging. In addition, some of the membrane element downstream should be left uncoated since the reaction of the coated reactive metal with oxidant impurities may provide coated material with poorer adhesion properties to the membrane support. This would lead to shedding and particulating. The uncoated membrane material downstream would be available for filtering these particles, but the pore size would be larger than optimal for reasons previously discussed. Efforts to improve pore size uniformity by decreasing the loading level of active material would result in a device with insufficient capacity for impurity removal. The efficiency and other characteristics of this membrane are unreported.
There exists a need for a purifier function integrated within an all-metal filter having good porosity and gas throughput. The integral combination of a getter purifier and a filter would allow for all of the positive aspects of highly porous metal filters and the gas impurity-removing capabilities of getters, with less outgassing and particulate shedding problems, in one point-of-use device.